Voltage Pro 1.0 | March 07, 2023

• 1 Introduction
  • 1.1 Purpose
  • 1.2 Integration with Plant Predict
  • 1.3 Codes and Standards Compliance
  • 1.4 References
  • 1.5 Terms of Use
• 2 Using Voltage Pro
  • 2.1 Accessing Voltage Pro
  • 2.2 Creating, Editing and Cloning a Project
  • 2.3 Creating, Editing and Cloning String Sizing Calculations
  • 2.4 Selecting Weather Data
  • 2.5 Selecting PV Module Data
  • 2.6 Selecting Inverter Data
  • 2.7    Modifying Calculation Settings
  • 2.8 Running the Analysis
  • 2.9 Accessing Results
  • 2.10 Safety Factors
  • 2.11 Generating Reports
  • 2.12 Accessing the Prediction in Plant Predict
• 3 Models & Algorithms
  • 3.1 Plant Predict Models & Algorithms
  • 3.2 Method 1 Calculation (Simplified Linear Approximation)
  • 3.3 Method 2 Calculation (Plant Predict Advanced Simulation)
  • 3.4 Default System Configuration
  • 3.5 Determination of P100 and P99.5 Voltages
  • 3.6 Determination of Open Circuit Cell Temperature
  • 3.7 Determination of Safety Factors

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1 Introduction

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1.1 Purpose

Voltage Pro is a cloud-based application that allows users to calculate the maximum string length (i.e., maximum number of modules in series) for a specific photovoltaic module at a specific project location. The application calculates maximum string length under two different methods:

• Method 1 – Simplified Linear Approximation
• Method 2 – Plant Predict Advanced Simulation

The application may be used to support both early-stage project development as well as detailed, construction-grade, engineering calculations.

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1.2 Integration with Plant Predict

The Voltage Pro application is tightly integrated with Plant Predict and is accessible with a PlantPredict license. Voltage Pro leverages the Plant Predict weather, module and inverter databases and uses the Plant Predict performance simulation capabilities to calculate open circuit voltages for each time step of a given prediction. When a Project is created in Voltage Pro, a Project of the same name and of type: StringSize is created in Plant Predict. Similarly, when a calculation is created and run in Voltage Pro, a prediction of the same name is created in Plant Predict. The following diagram provides a simple illustration of the relationship:

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1.3 Codes and Standards Compliance

Several codes and standards accept and provide justification for simulation-based string sizing calculations. However, these codes and standards are generally vague in defining exactly how simulation-based calculations should be performed. Terabase, through consultation with industry stakeholders and review of available research, considers the Voltage Pro application to be compliant with the following codes and standards when used with appropriate input data:

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1.4 References

The simulation-based methodology used within Voltage Pro is based in part on the following IEEE Journal of Photovoltaics research: T. Karin and A. Jain, “Photovoltaic String Sizing Using Site-Specific Modeling,” in IEEE Journal of Photovoltaics, vol. 10, no. 3, pp. 888-897, May 2020, doi: 10.1109/JPHOTOV.2020.2969788.

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1.5 Terms of Use

The use of Voltage Pro is governed by the Plant Predict Terms of Use: Plant Predict Terms of Use

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2 Using Voltage Pro

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2.1 Accessing Voltage Pro

Voltage Pro is bundled with Plant Predict. Pro and Enterprise tier licenses of Plant Predict have full access to Voltage Pro while all other license tiers have read only access. The application may be accessed from within the Plant Predict left navigation panel as follow: Alternatively, the application may be accessed from within the Development Platform by navigating to Terabase Apps as follows:

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2.2 Creating, Editing and Cloning a Project

Within Voltage Pro, a Project must be created before a string sizing calculation can be performed. To create a new Project, click the Create a New Project button and enter the project name, latitude, and longitude. Once the location (latitude and longitude) has been recognized, click the Save button to create the Project. Once created, the Project name may be edited by clicking the Rename button. However, the location is locked and cannot be edited. The Project may be cloned or deleted by clicking the Clone or Delete buttons respectively. Note that creating a Project within Voltage Pro automatically creates a project of the same name and location within Plant Predict. The status of the Plant Predict project is automatically set to StringSize to differentiate from other projects created directly within Plant Predict.

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2.3 Creating, Editing and Cloning String Sizing Calculations

Once a Project has been created, a new String Sizing Calculation may be created within the Project by clicking the + Add a string calculation button: Clicking the + Add a string calculation button immediately opens the string calculation page where all the inputs and outputs may be accessed. The calculation is automatically named (e.g., SCalc_01) but may be edited by clicking the Rename button. Similarly, calculations may be cloned or deleted by clicking the Clone or Delete buttons respectively. This can be done from the either the calculation page or the project page. Multiple string sizing calculations may be created under a single Project by either creating a new calculation (clicking the + Add a string calculation button) or cloning an existing calculation (clicking the Clone button). Calculations may continue to be edited until they have been run. Once a string sizing calculation has been run, only the safety factors may be edited. Note that the creation of a string sizing calculation within Voltage Pro does not automatically create a prediction within Plant Predict. However, a prediction is created within Plant predict once a string sizing calculation is run.

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2.4 Selecting Weather Data

Weather data is critical to simulation-based string sizing calculations and Voltage Pro makes it easy to pull in weather-related data due to its integration with Plant Predict. From within the calculation page, click the Update button associated with Weather data: Three tabs are available within the Weather data input panel as follows:

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Meteorological data

The Meteorological data tab allows the user to select the weather data file that will be used in the simulation-based calculation. All weather data files accessible to the user from Plant Predict are also accessible within Voltage Pro. The My weather files toggle button limits the weather datasets to only those that are owned by the user within Plant Predict. New weather data files must be added within Plant Predict before they can be loaded into Voltage Pro. Note that Plant Predict supports single year, partial year, and multi-year data as well as hourly and sub-hourly data. Once selected, basic information about the weather dataset selected will be displayed in the input panel.

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Extreme annual min. Temperature

The Extreme annual min. Temperature tab allows the user to view the range of months and years of the dataset selected within the Meteorological data tab. It also allows users to deselect or remove certain years from the meteorological data to be used in the simulation. This can be done by deselecting checkbox next to the year and is often done to remove partial years of data. Finally, the average extreme annual min. temperature from the selected years of the meteorological dataset is displayed. This temperature will be used in the calculation of the certain safety factors.

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ASHRAE climactic design data

By default, temperature data from the ASHRAE weather station closest to the Project site is pulled into the calculation. Specifically, the extreme annual mean minimum dry bulb temperature is used in both the Method 1 and Method 2 calculations and is displayed within the input panel.

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2.5 Selecting PV Module Data

Module data is critical to simulation-based string sizing calculations and Voltage Pro makes it easy to pull in module data due to its integration with Plant Predict. From within the calculation page, click the Select PV module button associated with PV module: The PV module data tab allows the user to select the PV module data file that will be used in the Method 1 and Method 2 string sizing calculations. All module data files accessible to the user from Plant Predict are also accessible within Voltage Pro. The My PV modules toggle button limits the PV modules to only those that are owned by the user within Plant Predict. New PV module files must be added within Plant Predict before they can be loaded into Voltage Pro. Once selected, basic information about the PV module will be displayed in the input panel. Click the Save button to save the selected PV module within the calculation.

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2.6 Selecting Inverter Data

Inverter data is used within Voltage Pro to set the maximum DC system voltage level (although this can be overridden in the calculation Settings). The inverter selected will be used within the Plant Predict simulation but will not impact the open circuit voltage calculation of the PV module. From within the calculation page, click the Select inverter button associated with Inverter: The Inverter data tab allows the user to select the inverter data file that will be used in the Method 2 simulation. The inverter maximum DC voltage also sets maximum string open circuit voltage (although this can be overridden within the calculation Settings) for both Method 1 and Method 2 calculations. All inverter data files accessible to the user from Plant Predict are also accessible within Voltage Pro. The My inverters toggle button limits the inverters to only those that are owned by the user within Plant Predict. New inverter files must be added within Plant Predict before they can be loaded into Voltage Pro. Once selected, basic information about the inverter will be displayed in the input panel. Click the Save button to save the selected inverter within the calculation.

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2.7    Modifying Calculation Settings

Several calculation settings may be modified as follows:

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Thermal Model

Module voltage is heavily influenced by cell temperature. The cell thermal model within the Method 2 simulation may be selected as either the Heat Balance Model (default) or the Sandia Thermal Model (optional). The Heat Balance Model is the default cell thermal model for all Plant Predict energy predictions.

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Conductive and Convective Coefficients

The conductive and convective coefficients used by the cell thermal models may be adjusted. These are used within the Method 2 simulation for their respective thermal model. If the Heat Balance Thermal Model is selected, the Sandia coefficients will not impact the calculations in any way and vice-versa.

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Maximum System Voltage

By default, the maximum inverter voltage is used within the Method 1 and Method 2 calculations. However, this may be overridden where other DC components have a lower maximum DC voltage limitation or where additional conservatism is desired.

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ASHRAE extreme annual mean minimum dry bulb temperature

By default, the ASHRAE extreme annual mean minimum dry bulb temperature from the nearest ASHRAE station to the project site is loaded. This may be overridden by the user if desired.

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2.8 Running the Analysis

Once all inputs have been entered, enter the Results tab click the Run analysis button to begin the calculation.